1. Field of the Invention
This invention relates in general to solder printing apparatus for the manufacture of portable products and specifically to solder printing apparatus for processing flexible tapes.
2. Description of the Related Art
In the expanding world of consumer electronics, the manufacture of portable products such as cellular telephones and pagers requires that the product designers incorporate the maximum performance into the least space and at the lowest cost. To fulfill the performance requirements of such products, yet remain small and affordable, a blend of highly integrated silicon and software, chip packaging, circuit board technologies and flexible circuits are commonly used.
Integrated circuits are the greatest enabling technology for size reduction; however, even with greater integration, circuit board technology is arguably the single factor most fundamentally affecting the level of product densification.
Manufacturing electronic circuits on rigid printed circuit boards (PCBs) is well known. The PCB, for example, is a composite of glass and epoxy, polyamide, or similar dielectric. Rigid PCBs have provided an effective manufacturing method for consumer products including cellular telephones, pagers, watches and hand-held calculators, However, as miniaturization continues to be a driving force in the technology market, new methods are under investigation to determine how to reduce the thickness of PCBs and more importantly how to manufacture new PCB technologies reliably and at low cost. As a byproduct of these investigations, engineers have turned their interest to the use of flexible substrates for carrying a multiplicity of circuits that traditionally were carried by rigid PCBs. Such substrates are constructed of, for example, polyamide, polyester, or similar material, typically having a thickness in the range of 0.0254 millimeter (0.001 inch) to 0.127 millimeter (0.005 inch), and are readily available and cost effective. Film circuitry (also known as flexible circuits) is the key to solving the related mechanical and cost issues of the manufacture of small portable products. Its cost effective construction allows for fine lines and spacing, small via interconnects, and mechanical freedom for not only two-dimensional structure but a full range of three-dimensional possibilities.
Although flexible circuits have the attractive feature of being inexpensive, their lack of rigidity has made manufacturability a complex challenge. Presently, the manufacture of dense circuits, which include fine-pitch surface mount devices, is expensive and not easily repeatable in a reliable fashion over large volumes of flexible circuits. Due to the inherent flexibility of the flexible circuitry, it is very difficult to use standard manufacturing printing processes. Currently available solder stencil printing equipment is capable of running rigid substrates with or without a carrier (i.e. boat, pallet) by utilizing a conveyorized material handling system. Substrates are moved into a working area from an external location, positioned, printed and the completed circuits are removed by a conveyorized system. It is possible, but very difficult, to adapt a conventional printing equipment for flexible circuitry. The design will be complex in order to hold the flexible tape flat and allow for advancing and steering of the flexible tape during normal operation. In any precise continuous tape application it is required that the flexible tape is maintained under constant tension during handling and/or printing.
There are several key challenges relating to continuous flexible tape advancement, printing and surface mount device (SMD) assembly of flexible circuits. One challenge is the advancement of the flexible flexible tape accurately under low tension in a continuous format without having sprocket holes on the exterior of the active circuitry. Another challenge is diverting the flexible circuits before and after the one being printed away from the solder stencil. A third challenge is keeping the flexible tape flat while allowing for smooth flexible tape advancement and steering. Another challenge is using standard solder printing equipment with a minimum of modification. A last challenge is using easy and cost effective methods to install onto printers that have a standard conveyor system.
What is needed is a design consistent with the transport mechanics and dimensional stability that is required by the existing manufacturing process equipment which also meets the key challenges previously mentioned.